The largest source of uncertainty is factor fl, the fraction of habitable planets with life. . . . The fact that it did occur here doesn’t give us information about fl other than the fact that fl is not exactly zero due to anthropic bias—the observation that we exist would be the same whether life on Earth was an incredibly rare accident or whether it was inevitable.
I think the latter sentence here is a strong claim and a controversial assumption. In particular, I disagree; I favor the self-indication assumption and its apparent implication that we should weight a possible universe by the number of experiences identical to ours in it, so (roughly) weight a possible universe by the number of planets in it where human-level civilization appears.
“Experiences identical to ours” includes “experiences of not seeing lots of alien civilizations”, which means that universes obviously teeming with life should be excluded, not weighted more highly. Assuming you favour SIA.
You’re right that we exclude universes obviously teeming with life. But we (roughly) upweight universes with lots of human-level civilizations that don’t see each other, or where civilizations that don’t see another are likely to appear.
As a simplified example, suppose we have prior reason to suspect that there is a single per-universe parameter p giving an independent per planet chance of technological civilization arising within 10 billion years or so that is log-uniform over [10^-100, 10^-0].
A civilization makes the Fermi observation: that they are not themselves (yet) starfaring and see no evidence of civilizations elsewhere. What update should they make to get a posterior credence distribution for the parameter?
By what criteria should we judge it to be a correct update?
The distribution of number of such civilizations per galaxy doesn’t much depend upon p for values above 10^-11 or so, but drops off quite linearly below that.
I think the latter sentence here is a strong claim and a controversial assumption. In particular, I disagree; I favor the self-indication assumption and its apparent implication that we should weight a possible universe by the number of experiences identical to ours in it, so (roughly) weight a possible universe by the number of planets in it where human-level civilization appears.
“Experiences identical to ours” includes “experiences of not seeing lots of alien civilizations”, which means that universes obviously teeming with life should be excluded, not weighted more highly. Assuming you favour SIA.
You’re right that we exclude universes obviously teeming with life. But we (roughly) upweight universes with lots of human-level civilizations that don’t see each other, or where civilizations that don’t see another are likely to appear.
Yes, that’s true.
As a simplified example, suppose we have prior reason to suspect that there is a single per-universe parameter p giving an independent per planet chance of technological civilization arising within 10 billion years or so that is log-uniform over [10^-100, 10^-0].
A civilization makes the Fermi observation: that they are not themselves (yet) starfaring and see no evidence of civilizations elsewhere. What update should they make to get a posterior credence distribution for the parameter?
By what criteria should we judge it to be a correct update?
The distribution of number of such civilizations per galaxy doesn’t much depend upon p for values above 10^-11 or so, but drops off quite linearly below that.